Flight Demonstration Of Low Overpressure N-Wave Sonic Booms And Evanescent Waves

2005·0.12 MB·English

by  Haering, Edward A., Jr.

Flight Demonstration Of Low Overpressure
N-Wave Sonic Booms And Evanescent Waves
Edward A. Haering, Jr.*, James W. Smolka†, James E. Murray*,
and Kenneth J. Plotkin¶
*Research Aerodynamics, NASA Dryden Flight Research Center, Edwards, CA 93523, USA
†Research Test Pilot, NASA Dryden Flight Research Center, Edwards, CA 93523, USA
¶ Chief Scientist, Wyle Laboratories, Inc., Arlington, VA 22202, USA
Abstract. The recent flight demonstration of shaped sonic booms shows the potential for quiet
overland supersonic flight, which could revolutionize air transport. To successfully design quiet
supersonic aircraft, the upper limit of an acceptable noise level must be determined through
quantitative recording and subjective human response measurements. Past efforts have
concentrated on the use of sonic boom simulators to assess human response, but simulators often
cannot reproduce a realistic sonic boom sound. Until now, molecular relaxation effects on low
overpressure rise time had never been compared with flight data. Supersonic flight slower than
the cutoff Mach number, which generates evanescent waves, also prevents loud sonic booms
from impacting the ground. The loudness of these evanescent waves can be computed, but flight
measurement validation is needed. A novel flight demonstration technique that generates low
overpressure N-waves using conventional military aircraft is outlined, in addition to initial
quantitative flight data. As part of this demonstration, evanescent waves also will be recorded.
Keywords: Sonic Boom, Flight Data, Evanescent Wave
PACS: 43.25.Cb, 43.28.Gq, 43.28.Mw, 43.50.Lj, 43.50.Rq, 43.50.Yw
HYPOTHESIS
The inspiration for producing low overpressure N-wave sonic booms originated
with the recent measurement of a sonic boom generated by a sounding rocket upon
descent[1]. This vehicle was in a very steep dive at a high altitude and low Mach
number when it generated the sonic boom that hit the recorder. Sonic booms of this
type were desired for recording and analysis, so the sonic boom propagation code
PCBoom4[2] was used to look at similar trajectories. Because additional flights of
this sounding rocket would be infrequent or nonexistent, alternative available aircraft
trajectories were modeled with a multitude of PCBoom4 runs. An aircraft in a steep
dive at a high supersonic Mach number was found to generate low overpressures far
forward of the dive point. It is hypothesized that these low amplitude booms could be
used for human acceptability studies leading to a supersonic aircraft quiet enough for
overland flight.
Four attributes have been determined to contribute to low overpressure N-waves
generated by conventional aircraft. The first attribute is a very long propagation
distance. High vehicle altitude increases propagation distance. The maximum
propagation distance occurs when the ray-path extends to near the lateral cutoff.
The second attribute is the use of shock waves coming from the top of the vehicle.
Near-field shock wave probing above and below a B-58 aircraft[3] shows that the
shock waves coming from the top of the vehicle are less than those coming from the
bottom because of the lift distribution around the aircraft. For the upper surface shock
waves to reach the ground (as the primary sonic boom, excluding “over-the-top”
booms), the vehicle must be in a dive. An additional benefit of the dive is that less lift
is generated on the aircraft, causing less intense shock waves caused by lift.
The third attribute of low overpressure N-waves is the use of the smallest vehicle
possible. Although an F/A-18B aircraft was used in this effort, an F-16 aircraft would
produce a smaller boom, because it has less boom due to volume. A T-38 aircraft also
was considered, but it has the disadvantages of less avionics to aid the pilot in the
maneuver and engines that are less forgiving in the high altitude supersonic flight
regime. The F/A-18B aircraft was selected, because it was readily available at the
NASA Dryden Flight Research Center (Edwards, California), and it had been
equipped with the appropriate instrumentation.
The fourth and last attribute is the minimization of Mach number. When a
particular Mach number and dive angle was found to produce the desired boom levels
on the ground, both Mach number and dive angle could be reduced while maintaining
a constant shock wave direction, thus preserving the long propagation path. A lower
supersonic Mach number and dive angle also created a condition in which the
maneuver was easier to achieve, and more importantly, easier to recover from.
A way to prevent the sonic boom from reaching the ground is to fly slower than the
Mach cutoff condition (in the range of Mach 1.1 to 1.3) such that the shock wave
refracts up away from the ground. On the ground, only an evanescent wave may be
heard, and if heard, would sound much like distant thunder. Flight research on
evanescent waves was conducted more than 30 years ago[4], but no high-fidelity
recordings were acquired that can be used in human acceptability studies. When they
are acquired, such recordings then can be used to validate computational models[5].
EXPERIMENT SETUP
A flight research project called the Low Boom/No Boom Experiment has been
initiated. Four flight phases are planned for this project, but the overarching objective
is to aid in the determination of an acceptable noise level for certifiable overland
supersonic vehicles. Table 1 presents the specific objective of each flight phase.
TABLE 1. Low Boom/No Boom Experiment Objectives.
Flight Low Boom No Boom
Phase (low overpressure N-waves) (evanescent waves)
1 Assess feasibility and repeatability of Gather limited digital high-fidelity
generating low overpressure N-waves microphone data for validation of
(<0.6 lbf/ft2) in a specified geographic area evanescent wave loudness predictions
2 Gather high-fidelity, statistically significant data for loudness levels and operational
considerations (atmospheric variability effects)
3 Perform outdoor human response surveys with flights within the restricted airspace
4 Perform outdoor human response surveys with flights outside the restricted airspace
(populated areas)
To implement the first phase of this experiment, a NASA Dryden F/A-18B aircraft
equipped with several instrumentation packages is employed. A Research Quick Data
System (RQDS) acts as the 1553 bus monitor of the aircraft parameters, including
airdata and inertial navigation system parameters, which are telemetered to the ground
for recording and real-time sonic boom footprint calculations. A time code generator
with an embedded global positioning system (GPS) receiver is used to time-stamp the
RQDS data.
In the transonic flight region the F/A-18B aircraft is known to have airdata position
errors[6] on the order of 2,000 ft of altitude. Additionally, the steep dives that are
performed may induce significant pneumatic lag in the aircraft airdata system. To
provide the necessary additional calibration for position error and lag, a carrier-phase
differential GPS receiver (that provides accurate inertial position and speeds) was
installed in the aircraft. The initiation of the supersonic dive involves rolling the
aircraft to an inverted attitude before diving, causing the GPS to lose data. For these
maneuvers, ground-based radar tracking was obtained using a C-band radar beacon on
the aircraft.
For the airdata calibrations and PCBoom4 sonic boom propagation codes, accurate
measurements of the atmospheric conditions are needed. The GPS rawinsonde
balloons and wind profiler data are used for atmospheric analysis. Some of these
sensors will be deployed at the location of the microphone array.
Microphones were placed over many miles of relatively flat desert terrain under a
supersonic corridor. The spacing of the microphones was chosen to obtain a range of
overpressures while allowing for atmospheric variability in footprint location. Several
NASA Dryden Boom Amplitude and Shape Sensors (BASS) and Boom Amplitude
and Direction Sensors (BADS[7]) were used to record the low booms in phase 1. A
few Brüel and Kjær (Denmark) 4193 microphones will be used to record evanescent
waves for phase 1.
Various dive profiles first were practiced in the NASA Dryden F/A-18 flight
simulator. The current dive profile involves flying at a level attitude, high subsonic
speed, and altitude of nearly 50,000 ft. The aircraft is rolled to an inverted attitude; a
positive g pull to the desired dive angle of 53° downward then is initiated, while the
throttle is pulled to the idle position to avoid excessive speed. When the desired dive
angle is reached, the aircraft is rolled to an upright attitude, and a Mach number of
approximately 1.1 is achieved. At an altitude of 38,000 ft a pull-up is executed to
recover the aircraft at an altitude of approximately 32,000 ft. The F/A-18B aircraft
has an angle-of-attack limit in this supersonic flight regime, so angle of attack is
closely monitored. The F/A-18 avionics allows a dive point to be displayed on the
head-up display (HUD), which greatly aids in maintaining the proper dive angle and
heading. The PCBoom4 runs of the simulation data predicted overpressures down to
approximately 0.2 lbf/ft2.
INITIAL RESULTS
The supersonic dives were practiced during a checkout flight of the RQDS system
on the F/A-18B aircraft. At the same time, the BASS recorders underwent field
checkouts while candidate microphone locations were sought. Although the surface
weather was sometimes quite breezy, low overpressure N-wave sonic booms were
recorded on multiple dives. Figure 1 shows two sets of these sonic boom
measurements.
1.0 1.2
0.8 1.0
2 0.6 2 0.8
bf/ft 0.4 bf/ft 0.6
e, l 0.2 e, l 0.4
r r 0.2
u 0 u
ss ss 0
e –0.2 e
pr pr–0.2
er –0.4 er –0.4
v v
O –0.6 O –0.6
–0.8 –0.8
–1.0 –1.0
0 0.04 0.08 0.12 0.16 0.20 0 0.04 0.08 0.12 0.16 0.20
Time, s Time, s
050218
FIGURE 1. Initial ground-level sonic boom measurements from the F/A-18B aircraft in a supersonic
low boom dive. Dive no. 3 is on the left, and dive no. 5 is on the right (April 1, 2005).
The microphones were spaced apart over 6 miles parallel to the ground track, and
an additional microphone was placed 1 mile off of the track. The ground sensor sites
located farthest from the aircraft generally show the lowest overpressure, and
overpressure increases as the dive location is approached. Atmospheric turbulence is
evident with repeated artifacts in each bow and tail shock. The rise time of the
signature greatly increases with decreasing maximum overpressure, as much as 40 ms
for one signature. The airdata calibration and lag determination has not yet been
completed, and no radar tracking or local weather balloon data were obtained from
this flight, but the experiment participants were very pleased to have obtained low
overpressure N-waves on the first attempt, even in nonoptimal weather conditions.
Such low overpressure sonic booms bracket the probable acceptable noise level and
can be used for human subjective testing.
REFERENCES
1. Plotkin, K. J., Haering, E. A., Jr., and Murray, J. E., “Low-Amplitude Sonic Boom From a Descending
Sounding Rocket,” Proceedings of the 17th International Symposium on Nonlinear Acoustics, American
Institute of Physics, Melville, New York, July 2005.
2. Plotkin, K. J., and Grandi, F., Computer Models for Sonic Boom Analysis: PCBoom4, CABoom, BooMap,
CORBoom, Wyle Report WR 02-11, June 2002.
3. Maglieri, D. J., Ritchie, V. S., and Bryant, J. F., Jr., In-Flight Shock-Wave Pressure Measurements Above and
Below a Bomber Airplane at Mach Numbers from 1.42 to 1.69, NASA TN D-1968, 1963.
4. Haglund, G. T., and Kane, E. J., Flight Test Measurements and Analysis of Sonic Boom Phenomena Near the
Shock Wave Extremity, NASA CR-2167, 1973.
5. Fung, K. Y., "Shock Wave Formation at a Caustic," SIAM Journal of Applied Mathematics, Vol. 39, No. 2, pp.
355–371, Oct. 1980.
6. Haering, E. A., Jr., Airdata Measurement and Calibration, NASA TM-104316, 1995.
7. Plotkin, K. J., Haering, E. A., Jr., Murray, J. E., Maglieri, D. J., Salamone, J., Sullivan, B. M., and Schein, D.,
“Ground Data Collection of Shaped Sonic Boom Experiment Aircraft Pressure Signatures,” AIAA-2005-010,
Jan. 2005.